MOUNT PLEASANT — More than 18 acres of invaluable shorebird rookery high ground has washed away from Crab Bank in Charleston Harbor over the past quarter-century. It hasn't gone far.

The sand has moved north toward the mouth of Shem Creek — a commercial fishing hub, tourist destination and restaurant row.

Getting in and out of the creek already means weaving around sand bars that occasionally strand boats.

Now there's a proposal to pile as much as 1.2 million cubic yards of soil from the $529 million harbor dredging as renourishment for the imperiled Crab Bank rookery, which has eroded to about only 1 acre of high ground.

The add-on could also be more than enough to silt in the mouth of Shem Creek completely — requiring more intensive dredging and possibly affecting the heavy traffic of fishing boats, pleasure boats and paddle craft coming in an out of the creek.

The mouth of the creek could be lost, said Randy Friedman, a Shem Creek property owner.

With the renourishment approval process underway, the potential for silting hasn't been fully studied, according to an engineer's analysis. It could cause the creek mouth to silt in faster.

The Army Corps of Engineers' Charleston District doesn't agree. Its engineers expect the bank will continue to erode as it has done since the island was initially formed by the placement of dredged material, spokeswoman Glenn Jeffries said.

Enough people are concerned that the town of Mount Pleasant formally requested the Army Corps to review the engineer's study.

"Obviously, the accumulation of silt deposited at the bottom of Shem Creek raises concerns about the future viability and uses of the creek," said Administrator Eric DeMoura.

Staffers from the Army Corps and the S.C. Department of Natural Resources met with town planners recently to review the project and the concerns. At that meeting the town asked for more information, DeMoura said. 

The analysis by Sam Phlegar, president of the Applied Technology and Management engineering firm in Mount Pleasant, found that the wash of tides and ship wakes has pushed the sand from Crab Bank 500 feet to the north since 1994. Adding dredging spoils to the bank will accelerate that drift and silt in the two channels to the creek, he said.

Phlegar recommends placing the dredging soil on the far end of the bank, toward the ocean, to mitigate that silting.

"Once Crab Bank continues to migrate north and eventually welds onto the mainland, it loses its habitat value as a rookery," the analysis said, because raccoons and other predators can easily get to its nests.

If more dredging is needed to keep the creek clear, that creates problems counted in dollars.

Crab Bank is one of only a handful of publicly owned and protected rookery islands in the state for threatened species of shorebirds. It is considered a critical mid-state rookery to protect and restore pelicans and other shorebirds that are among the eye-catching wildlife along the South Carolina coast.

The bank is a draw to a tourism economy worth billions of dollars per year. The dredging soil could rebuild it to 80 acres, with about 30 acres of nesting high ground. 

No one has ever put a dollar figure on the value of Shem Creek, but it's easily in the millions. The last time the creek and a channel leading to it were dredged, in 2014, shrimp boat captains said beforehand they couldn't get out of the creek at low tide. The dredging then cost $946,000. 

The creek mouth today "is already silted badly; I don't know how it could get much worse," said Anthony Noury, of Sea Tow Charleston.

The silting shifts the channels month to month, and the company regularly pulls boats off the sand, he said.

With current budget and fundraising crimps, the money to dredge is tight. The Army Corps district has to compete for it, and a local share is required.

The local office is trying to win competitive federal funding to pay for its share of the prospective $3 million cost to renourish the bank.

The Coastal Bird Conservation Partners, a coalition of private and public groups organized to push for the renourishment, has to come up with $1.25 million to pay its share.

The coalition is about to launch a public fundraising effort. One of its members conceded frankly more dredging might be needed around the bank.

"The reality is, Shem Creek has always been hydrologically managed (in modern times)," said Sharon Richardson, executive director of Audubon South Carolina, the member partner.

Reach Bo Petersen Reporter at Facebook, @bopete on Twitter or 1-843-937-5744.

Science and environment reporter. Author of Washing Our Hands in the Clouds.